.\"	$OpenBSD: SSL_shutdown.3,v 1.5 2018/03/27 17:35:50 schwarze Exp $
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.Dd $Mdocdate: March 27 2018 $
.Dt SSL_SHUTDOWN 3
.Os
.Sh NAME
.Nm SSL_shutdown
.Nd shut down a TLS/SSL connection
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft int
.Fn SSL_shutdown "SSL *ssl"
.Sh DESCRIPTION
.Fn SSL_shutdown
shuts down an active TLS/SSL connection.
It sends the
.Dq close notify
shutdown alert to the peer.
.Pp
.Fn SSL_shutdown
tries to send the
.Dq close notify
shutdown alert to the peer.
Whether the operation succeeds or not, the
.Dv SSL_SENT_SHUTDOWN
flag is set and a currently open session is considered closed and good and will
be kept in the session cache for further reuse.
.Pp
The shutdown procedure consists of 2 steps: the sending of the
.Dq close notify
shutdown alert and the reception of the peer's
.Dq close notify
shutdown alert.
According to the TLS standard, it is acceptable for an application to only send
its shutdown alert and then close the underlying connection without waiting for
the peer's response (this way resources can be saved, as the process can
already terminate or serve another connection).
When the underlying connection shall be used for more communications,
the complete shutdown procedure (bidirectional
.Dq close notify
alerts) must be performed, so that the peers stay synchronized.
.Pp
.Fn SSL_shutdown
supports both uni- and bidirectional shutdown by its 2 step behavior.
.Pp
When the application is the first party to send the
.Dq close notify
alert,
.Fn SSL_shutdown
will only send the alert and then set the
.Dv SSL_SENT_SHUTDOWN
flag (so that the session is considered good and will be kept in cache).
.Fn SSL_shutdown
will then return 0.
If a unidirectional shutdown is enough
(the underlying connection shall be closed anyway), this first call to
.Fn SSL_shutdown
is sufficient.
In order to complete the bidirectional shutdown handshake,
.Fn SSL_shutdown
must be called again.
The second call will make
.Fn SSL_shutdown
wait for the peer's
.Dq close notify
shutdown alert.
On success, the second call to
.Fn SSL_shutdown
will return 1.
.Pp
If the peer already sent the
.Dq close notify
alert and it was already processed implicitly inside another function
.Pq Xr SSL_read 3 ,
the
.Dv SSL_RECEIVED_SHUTDOWN
flag is set.
.Fn SSL_shutdown
will send the
.Dq close notify
alert, set the
.Dv SSL_SENT_SHUTDOWN
flag and will immediately return with 1.
Whether
.Dv SSL_RECEIVED_SHUTDOWN
is already set can be checked using the
.Fn SSL_get_shutdown
(see also the
.Xr SSL_set_shutdown 3
call).
.Pp
It is therefore recommended to check the return value of
.Fn SSL_shutdown
and call
.Fn SSL_shutdown
again, if the bidirectional shutdown is not yet complete (return value of the
first call is 0).
.Pp
The behaviour of
.Fn SSL_shutdown
additionally depends on the underlying
.Vt BIO .
.Pp
If the underlying
.Vt BIO
is
.Em blocking ,
.Fn SSL_shutdown
will only return once the
handshake step has been finished or an error occurred.
.Pp
If the underlying
.Vt BIO
is
.Em non-blocking ,
.Fn SSL_shutdown
will also return when the underlying
.Vt BIO
could not satisfy the needs of
.Fn SSL_shutdown
to continue the handshake.
In this case a call to
.Xr SSL_get_error 3
with the
return value of
.Fn SSL_shutdown
will yield
.Dv SSL_ERROR_WANT_READ
or
.Dv SSL_ERROR_WANT_WRITE .
The calling process then must repeat the call after taking appropriate action
to satisfy the needs of
.Fn SSL_shutdown .
The action depends on the underlying
.Vt BIO .
When using a non-blocking socket, nothing is to be done, but
.Xr select 2
can be used to check for the required condition.
When using a buffering
.Vt BIO ,
like a
.Vt BIO
pair, data must be written into or retrieved out of the
.Vt BIO
before being able to continue.
.Pp
.Fn SSL_shutdown
can be modified to only set the connection to
.Dq shutdown
state but not actually send the
.Dq close notify
alert messages; see
.Xr SSL_CTX_set_quiet_shutdown 3 .
When
.Dq quiet shutdown
is enabled,
.Fn SSL_shutdown
will always succeed and return 1.
.Sh RETURN VALUES
The following return values can occur:
.Bl -tag -width Ds
.It 0
The shutdown is not yet finished.
Call
.Fn SSL_shutdown
for a second time, if a bidirectional shutdown shall be performed.
The output of
.Xr SSL_get_error 3
may be misleading, as an erroneous
.Dv SSL_ERROR_SYSCALL
may be flagged even though no error occurred.
.It 1
The shutdown was successfully completed.
The
.Dq close notify
alert was sent and the peer's
.Dq close notify
alert was received.
.It \(mi1
The shutdown was not successful because a fatal error occurred either
at the protocol level or a connection failure occurred.
It can also occur if action is need to continue the operation for non-blocking
.Vt BIO Ns
s.
Call
.Xr SSL_get_error 3
with the return value
.Fa ret
to find out the reason.
.El
.Sh SEE ALSO
.Xr BIO_new 3 ,
.Xr ssl 3 ,
.Xr SSL_accept 3 ,
.Xr SSL_clear 3 ,
.Xr SSL_connect 3 ,
.Xr SSL_CTX_set_quiet_shutdown 3 ,
.Xr SSL_free 3 ,
.Xr SSL_get_error 3 ,
.Xr SSL_set_shutdown 3
.Sh HISTORY
.Fn SSL_shutdown
first appeared in SSLeay 0.8.0 and has been available since
.Ox 2.4 .
